Field of the Invention
This invention relates generally to thermal energy collection and storage systems, and more particularly to portable thermal energy collection and storage systems.
Description of the Background Art
Thermal energy storage systems are a fast growing and evolving technology. Some systems convert solar energy into thermal energy and store it in some type of thermal storage medium (e.g., NaNO2, NaNO3, CaCl2, LiF, KNO3, etc.). Typically, this is done in large facilities that provide energy to multiple consumers. Such facilities often employ several light focusing devices, both light permissive (e.g. Fresnel lens) and reflective (e.g. parabolic mirror), to heat some type of large thermal storage device such as, for example, a fluid reservoir, a pipe network, etc.
Although such facilities provide useful clean energy, they are stationary and, therefore, can only provide energy to consumers that are located relatively close to the facility. In an effort to provide energy to more remote locations, portable thermal energy storage systems have been developed. Such systems typically include a small light focusing device that heats a discrete thermal storage device via sunlight. Additionally, such systems employ some suitable sun tracking system that adjusts the position of the light focusing device and thermal storage device to accommodate for the continuous change in position of the sun. That is, the sun tracking system assures that the optical axis is pointed at the sun throughout the day. In doing so, the thermal storage device is typically tilted, turned, lifted, and/or lowered throughout the day.
There are several challenges with current portable thermal energy storage systems. For example, thermal storage devices are inherently massive because the amount of heat storage is proportional to the amount of material in which the heat is stored. Consequently, mounting a massive storage unit at the focal point of a light focusing device would impose undesirable strength and rigidity requirements on the tracking system and also increase the tracking power required to vary the vertical height of the thermal storage device. As another example, there are undesirable consequences of tilting thermal storage devices. Because the orientation of a storage container changes throughout the day, any such mounting requires that the storage container be sealed to prevent loss of the storage medium. Some storage media (such as salts or aluminum) can be corrosive. As the container's orientation with respect to the vertical is changed, that medium will come into contact with the sealed lid and the joint between the lid and container, which can cause failure of the seal/joint. Furthermore, should the heat storage mechanism (such as a phase change) involve changes in volume of the heat storage medium, any lid sealing technique must be sufficiently strong to withstand the forces resulting from this volume change. Ultimately, the demands on the container and lid become difficult to meet.
What is needed, therefore, is a portable thermal storage system that does not require changing the vertical height of the thermal storage device to maintain optical alignment between the solar energy concentrator and the thermal storage device as the position of the sun changes. What is also needed is a portable thermal storage system that does not require tilting the thermal storage device to maintain optical alignment between the solar energy concentrator and the thermal storage device as the position of the sun changes.